Communication on a wide band channel requires a passband filter at the input of the signal. The response of the passband filter should accurately receive the wideband signal making a discriminator type demodulator suitably efficient for decoding the wideband signal. However, when the signal to be received comprises a plurality of narrowband signals within a single wideband channel, the plurality of narrow band signals are decoded by using a plurality of filters having a response tuned to each of the frequencies. The plurality of filters provide a very cost intensive decoding technique.
Unfortunately, with communication on a narrow band channel, the received signal suffers from two major problems: fading and frequency offset. With fading, the envelope and the phase of the received signal vary in a random manner because of multi-path effects. If a transmitted frequency modulated (FM) signal has a bandwidth less than the coherence bandwidth of the multipath channel, the random FM noise produced by the received signal phase variations will be superimposed on the desired modulation. Therefore, when a conventional discriminator is used to demodulate the signal, the random FM places an upper boundary on the baseband signal-to-noise ratio achievable at the output of the frequency discriminator. Also, the upper bound of the signal-to-noise ratio (ceiling) gets lower for a narrow band signal. This ceiling results in a floor in the error rate for the demodulation.
Additionally, the narrow band signals have a random frequency offset causing the narrow band signals to be received at different positions within their subchannels. Therefore, the plurality of narrow bandpass filters must have a frequency response at least equal to each of narrow band subchannels. By increasing the bandwidth of the narrow bandpass filters, the signals will arrive within the bandwidth of the narrow bandpass filters. Unfortunately, to guarantee the reception of each narrow band signal, the signal to noise ratio of the filter must be severely degraded.
Thus what is needed is a method and apparatus for receiving and decoding the plurality of narrow band signals for generating a high signal to noise ratio in a cost effect manner.